<!DOCTYPE html>
<style>
.box {
width: 100px;
height: 100px;
border-radius: 50%;
overflow: hidden;
box-sizing: border-box;
}
.inner-box {
width: 100px;
height: 100px;
background-color: magenta;
}
#container {
width: 200px;
height: 200px;
border: 1px solid black;
}
</style>
<div id="container">
<div class="box" id="a"><div class="inner-box" id="aa"></div></div>
<div class="box" id="b"><div class="inner-box" id="bb"></div></div>
</div>